Hitherto, various processes for graining the surface of an aluminum plate composed of aluminum or aluminum alloy suitable for use as a support for lithographic printing plates have been known. For example, there are mechanical graining processes such as a ball graining, wire graining, brush graining, etc., chemical graining processes, and electrochemical processes such as an electrolytic graining process. After the surface of the aluminum is subjected to graining processing by one or a combination of these graining processes so as to form a roughened surface, it is etched with an aqueous solution of acid or alkali. After it is subjected to anodic oxidation, it is processed, if desired, so as to have a hydrophilic property to produce an aluminum support for lithographic printing plates.
On the resulting support, a lithographically suitable light-sensitive layer is provided to produce a light-sensitive lithographic printing plate (the so-called PS (Pre-Sensitized) plate). This PS plate is generally subjected to steps of imagewise exposure, development, retouching and gumming, etc., to produce a lithographic printing plate, which is then placed on a printing machine to carry out printing.
It must be noted that the graining of the surface of the aluminum plates by the above-described mechanical graining processes does not bring about satisfactory results in many uses for lithographic printing, because a comparatively coarse and unevenly grained surface is formed.
On the other hand, according to the electrolytic graining process, the same grained surface as that in case of mechanical graining is formed during electrolytic processing. However, if the electrolytic processing is continued, secondary pits are formed in the first-formed primary pits, to obtain an aluminum plate having the so-called double-structure or pits-in-pit grained surface. Accordingly, the lithographic printing plate using such an aluminum plate as a support is unsatisfactory in press life of the plate, or is unsatisfactory from the viewpoint that the non-image area is easily contaminated, although it does have a remarkably improved printing performance. In addition, it has the fault that production thereof consumes large electricity. Thus, a process wherein formation of primary pits is carried out by mechanical graining and formation of secondary pits is carried out by electrolytic graining has been developed (for example, refer to British Patent No. 2,047,274). In this process, it is necessary that hemispherical pits having a small diameter are formed in high density in the secondary graining step by electrolysis, but sufficiently satisfactory results in press life and contamination have not been obtained, yet.
On the other hand, a process wherein a double structure grained surface is formed solely by the electrolytic graining processing is known. Namely, Japanese Patent Publication 51119/81 has disclosed a process which comprises in a nitric acid type electrolyte forming a primary pit structure by electrolyzing at a high current density as the first step, and forming secondary pits by electrolyzing at a low current density as the second step with a desmutting step therebetween. However, a lithographic printing plate using an aluminum plate grained in such a manner as a support has a disadvantage that the non-image area is easily contaminated.
Moreover, U.S. Pat. No. 4,072,589 has disclosed an electrolytic graining process for an aluminum plate which comprises electrolyzing with an alternating current at a specific current density in an electrolyte containing hydrochloric acid and nitric acid in a ratio by weight of from 1/4 to 1/6 at 40.degree. C. or more, but there is a problem in that a lithographic printing plate using the resulting aluminum support has inferior press life.